In conventional or “wet” lithographic printing, ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink, and the ink receptive regions accept the ink and repel the water. The ink is transferred to the surface of a material upon which the image is to be reproduced. For example, the ink can be first transferred to an intermediate blanket that in turn is used to transfer the ink to the surface of the material upon which the image is to be reproduced.
Waterless printing plates have been known and used since 1970. These printing plates can be used for printing without the need for dampening water (fountain solution) on-press. Most waterless printing plates involve an ink-repelling layer such as a silicone layer overlying a radiation-sensitive layer and substrate that are more ink-receptive. Some details of waterless printing and its advantages are provided, for example, at www.wateress.org that is a web site for the Waterless Printing Association.
For example, some of the benefits of waterless printing include consistent color in the image (better color fidelity), better color saturation, and lower dot gain (more detail). In addition, because a fountain solution is not used during printing, a greater variety of papers including uncoated papers, can be used in printing. Moreover, faster make-readies can be achieved with waterless printing and small compact printing presses can be used so that printing operations can be carried out in smaller facilities (less investment in equipment and buildings). By avoiding the use of a fountain solution, waterless printing is better for the environment. Printing operators no longer need to carefully balance fountain solution and lithographic printing ink and can carry out their operations with less training.
Positive-working waterless printing plates have been prepared from imageable elements containing a negative-working diazo resin and UV irradiation. Negative-working waterless printing plates have been obtained using UV irradiation and diazonaphthoquinone-containing imageable elements or acid-catalyzed chemistries.
The majority of the early waterless printing plates were prepared using a photographic film. The use of such films is expensive and tedious. These disadvantages were addressed in recent years with “computer-to-press” (CTP) technologies whereby waterless printing plates are imaged directly by computer-generated signals using one or more lasers.
One method for preparing waterless printing plates includes generating a contact mask on a radiation-sensitive imageable element. The mask can be produced, for example, using a digital device such as an inkjet printer, electrographic printer, or any other apparatus containing a digitally-controlled laser. Laser ablation, laser ablative transfer, or laser-induced color change techniques can also be used to produce the mask. However, the use of masks to produce waterless printing plates is expensive and requires complicated processing methods.
Two commercial types of thermal (computer-to-plate, or “CTP”) waterless printing systems are known. One waterless printing system includes imaging by laser ablation that includes a destructive breaking away or volatilization and removal of the matter in the imaged layer(s). Ablative imaging requires very high imaging energy (the imageable elements have relatively slow imaging speed) and creates considerable debris and gaseous effluents that must be captured or released into the environment. U.S. Pat. No. 5,339,737 (Lewis et al.) and U.S. Pat. No. 5,353,705 (Lewis et al.) describe multi-layer ablatable elements and imaging systems for making waterless printing plates.
Another waterless printing system requires thermal imaging with a laser (perhaps through a mask) to solubilize imaged layers that are then removed in imaged regions by using a developer that often contains an organic solvent or by using a pretreatment solution, or both, as described, for example in U.S. Pat. No. 4,342,820 (Kinashi et al.), U.S. Pat. No. 6,074,797 (Suezawa et al.), U.S. Pat. No. 6,284,433 (Ichikawa et al.), and U.S. Pat. No. 6,964,841 (Tihara et al.).
U.S. Pat. No. 5,919,600 (Huang et al.) describes a two-layer non-ablative imageable element that can be used to provide waterless printing plates using solvent-containing developers to remove the solubilized imageable layer and overlying silicone layer.
Problem to be Solved
Waterless printing is a desired printing technique that can be used without a fountain solution and provide several advantages. Thus, there is a need for imageable elements that can be used to provide waterless printing plates without ablation imaging and development using developers that contain predominantly organic solvents. It is also desired that when silicone is removed from the element during development, silicone debris removed in small pieces that do not clog the processor.